linux-kernel - Re: [PATCH 1/5] workqueue: Allow changing attributions of ordered workqueues

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Message-ID: <20140520150842.GD17741@localhost.localdomain>
Date:	Tue, 20 May 2014 17:08:43 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Kevin Hilman <khilman@...aro.org>,
	Mike Galbraith <bitbucket@...ine.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH 1/5] workqueue: Allow changing attributions of ordered
 workqueues

On Tue, May 20, 2014 at 10:35:34AM -0400, Tejun Heo wrote:
> Hello,
> 
> On Tue, May 20, 2014 at 04:32:31PM +0200, Frederic Weisbecker wrote:
> > > But that's the same for other pwqs too.  Back-to-back requeueing will
> > > hold back pwq switching on any workqueue.
> > 
> > I don't think so, because non ordered pwqs aren't created with 0 max_active,
> > so they can run before the old pwq is released. It's not holding back the new
> > one and creating a starvation there.
> > 
> > But maybe I forget other details.
> 
> Ah, I was thinking about old pwq not being allowed to be released
> while one or more work items are requeueing themselves back-to-back.

Right. OTOH, if you have non-deffered work items requeuing themselves
back to back for ever, you may have a much bigger problem than just a few
unreleased bytes :)

> Yeap, the new ones can still be used for other work items.
> 
> > > Maybe I'm confused but I don't think it'd be.  Let's say there was an
> > > attribute change with one work item, A, which is performing
> > > back-to-back requeueing and another one, B, which queues itself
> > > intermittently.  If B is queued while A is executing, followed by A
> > > requeueing itself, the expected execution order is A - B - A; however,
> > > without the above exception for ordered workqueues, it'd end up A - A
> > > - B because B will end up on the new pwq while A on the older one and
> > > max_active won't be transferred to the new pwq before it becomes
> > > empty.
> > 
> > Ah right AAB instead of ABA is possible indeed. I don't know if some workqueue
> > rely on such guarantee but it's possible.
> 
> That's part of the ordering guarantee of ordered workqueues so we
> better not break it.

Ok.

> 
> > In which case we have one more reason to make an exception on ordered workqueues
> > previous pwq reuse.
> 
> Yeah, I agree the special treatment is necessary but the current
> comment is misleading.

Ok got it, I'll try to improve the comment.

Thanks.
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